Industrial gases which are utilized in small to moderate volumes are stored at ambient temperature in pressurized cylinders from which gas is withdrawn as needed. Gases which have critical temperatures below ambient temperature are stored at high pressures determined by the design pressure ratings of the cylinders. Examples of these include low-boiling gases such as nitrogen, oxygen, hydrogen, helium, and methane, which are withdrawn from the storage cylinders without phase change. Gases which have critical temperatures above ambient temperature are stored in cylinders as compressed saturated liquids at their respective vapor pressures, and these liquids vaporize as saturated vapor is withdrawn from the cylinders. Common examples of such liquefied compressed gases are chlorine, ammonia, and light hydrocarbons such as propane and butane.
There is a need in certain industries for very high purity gases supplied by cylinders as described above. For example, very high purity gases are utilized in the electronics industry for manufacturing semiconductor and optoelectronic devices, video display panels, fiber optic devices, and micro-electromechanical systems. Examples of such gases include hydrogen chloride, hydrogen bromide, hydrogen fluoride, chlorine, ammonia, tungsten hexafluoride, disilane, dichlorosilane, trimethylsilane, and boron trichloride.
The demand for higher purity levels of these gases, especially those supplied as liquefied compressed gases, is growing. There is a trend towards the use of storage cylinders or vessels which have built-in purifiers within the cylinder or vessel that purify the stored gas as it is withdrawn, thereby providing an ultra-pure gas product. There is a need in the field of high purity gas supply systems for improved built-in purifiers having higher purification capacities and longer operating life. There also is a need for methods to isolate built-in purifiers from the gas during periods when gas is not being withdrawn through the purifiers. These needs are addressed by embodiments of the present invention descried below and defined by the claims that follow.